CN100416863C - Cheap Polysilicon Thin Film Solar Cells - Google Patents

Abstract

A cheap polysilicon thin film solar cell, the structure is back electrode/substrate/polysilicon thin film with PN junction/grid upper electrode, there is a silicon nitride anti-reflection film on the incident light surface of the cell, and it is characterized in that: on the substrate and Between the polysilicon films with PN junctions there is a heavily doped P ⁺ layer and an SiO ₂ isolation layer that blocks the diffusion of substrate impurities to the polysilicon films; the substrate is made of low-quality polysilicon wafers with a purity of <4N. The greatest advantage of the present invention is: between the substrate and the polysilicon thin film that is formed on PN junction, increase SiO ₂ isolation layer and heavily doped P ⁺ layer, make low-quality polysilicon chip substrate be used for solar cell smoothly, reduce greatly The production cost of the battery is conducive to the industrialization of thin-film solar cells, which in turn is conducive to the realization of low-cost photovoltaic power generation.

Description

Cheap Polysilicon Thin Film Solar Cells

technical field

The invention relates to a silicon solar cell, in particular to a polycrystalline silicon thin-film solar cell with cheap and low-quality polycrystalline silicon as a substrate.

Background technique

Polycrystalline silicon thin-film solar cells are considered to be the most likely to replace conventional bulk silicon solar cells due to their high efficiency potential and low cost. They have aroused great interest and been extensively studied in the past decade. The research of polycrystalline silicon thin film solar cells focuses on two aspects: one is the selection of the cell substrate, and the other is the preparation process. There are basically two technical routes for the preparation process of polycrystalline silicon thin film batteries: high temperature route and low temperature route. The high-temperature route refers to the method in which the film deposition temperature and the battery manufacturing process are greater than 800°C, and the low-temperature route refers to the thin-film deposition and battery process below 650°C. Different temperature ranges determine the different substrate materials used. Although low-temperature deposition has the advantage of low cost, the deposition rate is slow, the output is low, the film quality is not high, and the battery efficiency is low, which is very unfavorable to its industrialization development. High-temperature deposition of polysilicon films has good quality, fast deposition rate, and high cell efficiency, which can far compensate for the economic losses caused by high energy consumption. Many research institutions still use high-temperature deposition methods to prepare polysilicon films, especially in the chemical vapor deposition (CVD) complex area. The melting and recrystallization (ZMR) process is the mainstream of producing polysilicon thin films today. The highest efficiency of single-junction polycrystalline silicon thin-film solar cells obtained by this process reaches 19.2%. In order to prepare cost-effective polysilicon thin-film solar cells, the substrate material must not only meet the basic requirements of matching the thermal expansion coefficient with silicon, high mechanical strength and stable chemical properties, and not react with silicon at high temperatures, but also must be cheap. If we simply consider the performance of materials, high-purity, high-quality monocrystalline silicon wafers are undoubtedly very ideal substrate materials for polycrystalline silicon thin film solar cells, but they are expensive. In order to reduce the cost of photovoltaic power generation, it is necessary to select cheap substrate materials that meet the above basic performance requirements. However, during the high-temperature process, the impurities in the cheap substrate are easy to diffuse into the film, which will affect the performance of the battery and reduce the efficiency of the battery.

Contents of the invention

Aiming at the problems existing in the above-mentioned prior art, the object of the present invention is to provide a polysilicon thin-film solar cell with cheap and low-quality polysilicon as the substrate by improving the design of the cell structure, so as to reduce the cost of photovoltaic power generation.

The polysilicon thin-film solar cell of the present invention has a structure of back electrode/substrate/polysilicon thin film/gate-shaped upper electrode with a PN junction, and a silicon nitride anti-reflection film is arranged on the incident light surface of the cell, and is characterized in that: on the substrate There is also a heavily doped P ⁺ layer and a SiO ₂ isolation layer between the polysilicon film with a PN junction, and a window for the back electrode and the upper electrode to form an electrical circuit is opened in the SiO ₂ isolation layer; the substrate is selected For low-quality polysilicon wafers, the purity is less than 4N; the thickness of the SiO ₂ isolation layer is greater than or equal to 2 μm.

The purpose of using the SiO ₂ isolation layer is to block the diffusion of substrate impurities to the polysilicon film. Some windows are photoetched on the SiO ₂ isolation layer, so that the electrodes can be made on the front and back sides of the battery, which greatly simplifies the manufacturing process of the battery.

The greatest advantage of the present invention is: between substrate and the polysilicon thin film that is formed with PN junction, increase SiO ₂ spacer layer and heavily doped P ⁺ layer, make low-quality polysilicon chip substrate be used for solar cell smoothly, reduce greatly The production cost of the battery is conducive to the industrialization of thin-film solar cells, which in turn is conducive to the realization of low-cost photovoltaic power generation.

Description of drawings

FIG. 1 is a schematic structural view of the polycrystalline silicon thin film solar cell of the present invention.

Detailed ways

The specific embodiment of the present invention is described in further detail below in conjunction with accompanying drawing:

(1) Substrate 2 pretreatment: select heavily doped low-quality polysilicon wafers with a purity of <4N, use semiconductor cleaning technology to clean the substrate surface initially, etch the silicon wafers with sodium hydroxide to remove mechanical cuts on the surface of the silicon wafers. To remove scratches and damages, remove the silicon dioxide layer on the surface with 5% hydrofluoric acid, then clean the substrate several times with deionized water, and dry it with nitrogen gas.

(2) Plasma-enhanced chemical vapor deposition (PECVD) deposition of silicon dioxide isolation layer 3: using SiH ₄ and N ₂ O as reaction gases, a layer of SiO ₂ isolation layer with a thickness greater than or equal to 2 μm was deposited by PECVD.

(3) Photoetch some windows 301 on the SiO ₂ isolation layer. The windows should not be too large to prevent substrate impurities from diffusing into the polysilicon film. The window size should be 80 μm ² -100 μm ² , with an interval of 250 μm.

(4) Deposit heavily doped P ⁺ layer 4 by rapid thermal chemical vapor deposition (RTCVD): H ₂ , SiH ₂ Cl ₂ and B ₂ H ₆ are used as carrier gas, reactant and P-type dopant respectively, and RTCVD method is used Deposit a heavily doped P ⁺ layer polysilicon film with a thickness of 5-10 μm.

(5) Deposition of silicon dioxide protective layer by PECVD method: with SiH ₄ and N ₂ O as reaction gases, a layer of SiO ₂ protective layer with a thickness of about 2 μm was deposited by PECVD method.

(6) Zone melting recrystallization (ZMR): In the ZMR system, the lower surface of the silicon wafer is heated by the light radiation generated by the halogen tungsten lamp, and the temperature can quickly rise to 1000-1200 ° C. The upper surface of the sample is controlled by a polymer The linear beam formed by the tungsten-halogen lamp in the optical cavity is heated to form a linear melting zone on the surface of the sample. Through recrystallization, the grain size of the P ⁺ layer is increased, making it a P-type polysilicon film. the seed layer. Using it as the seed layer can increase the grain size of the deposited epitaxial P-type layer film, improve the performance of the film, and then improve the efficiency of the solar cell; the heavily doped P ⁺ layer can also form a back electric field with the epitaxial P-type layer, which can At the same time, the short-circuit current and open-circuit voltage of the battery are increased; moreover, the heavily doped P ⁺ layer has a certain gettering effect, which can prevent the diffusion of impurities to the P-type layer.

(7) Etching and removing the silicon dioxide protective layer.

(8) Deposit P-type polysilicon film 5 by RTCVD method: H ₂ , SiH ₂ Cl ₂ and B ₂ H ₆ are used as carrier gas, reactant and P-type dopant respectively, the deposition temperature is 1150-1200°C, and the concentration of boron doping is controlled At around 10 ¹⁶ cm ^-3 , the film thickness is 20-30 μm.

(9) Preparation of N-type diffusion layer 6 by thermal diffusion: POCl ₃ liquid source is used as the diffusion source, the diffusion temperature is 850-900°C, and the sheet resistance after diffusion is controlled at 55-65Ω/□.

(10) Electrode fabrication: Ti/Pd/Ag back electrode 1 is vacuum evaporated, the back electrode is sintered, and Ti/Pd/Ag upper electrode 7 is evaporated with a mask.

(11) Deposition of silicon nitride anti-reflection film 8: using (4.5% SiH ₄ +95.5% N ₂ ) and NH ₃ as reaction gases, using high-frequency (13.56Hz) PECVD process deposition, high-frequency power is 150W, the substrate The temperature is 400°C, and the thickness of the silicon nitride film is controlled at 70-75nm. The silicon nitride anti-reflection coating can reduce the reflection of light on the surface of the battery and improve the efficiency of the battery.

Claims (2)

1. Cheap polysilicon thin film solar cell, structure is followed successively by back electrode (1), substrate (2), is shaped on polysilicon membrane, the palisade top electrode (7) of P-N knot, on the incident light surface of battery silicon nitride antireflective coating (8) is arranged, it is characterized in that: at substrate (2) be shaped between the polysilicon membrane of P-N knot and also be shaped on SiO successively ₂Separator (3), heavy doping P ⁺Layer (4); At SiO ₂Have in the separator and can make back electrode and top electrode constitute the window (301) in electric loop; Said substrate (2) is to select inferior polysilicon chip for use, its purity＜4N.

2. according to a kind of Cheap polysilicon thin film solar cell of claim 1, it is characterized in that: said SiO ₂Separation layer thickness is more than or equal to 2 μ m.

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